Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Photovoltaic (PV) modules (also known as solar modules) are used to produce electrical energy from sunlight, offering a more environmentally friendly alternative to traditional methods of electricity generation. Such modules are based on a variety of semiconductor cell systems that can absorb light and convert it into electrical energy. These systems are typically categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based modules and thin film modules. Typically, individual cells are electrically connected in an array to form a module, and an array of modules can be connected together in a single installation to provide a desired amount of electricity.
When the light absorbing semiconductor material in each cell and the electrical components used to transfer the electrical energy produced by the cells are suitably protected from the environment, photovoltaic modules can last 25, 30, and even 40 or more years without significant degradation in performance. In a typical photovoltaic module construction, the solar cell layer is positioned between two encapsulant layers, which are further positioned between frontsheet and backsheet layers providing weather resistance, UV resistance, moisture barrier properties, low dielectric constant, and high breakdown voltage.
Fluoropolymer films are recognized as important components in photovoltaic modules due to their excellent strength, weather resistance, UV resistance, moisture barrier properties, low dielectric constant, and high breakdown voltage and can play a role in both wafer-based and thin film modules. For example, a fluoropolymer film, such as an ethylene-tetrafluoroethylene copolymer (ETFE) film, may be used as a frontsheet for a photovoltaic module instead of the more common glass layer. Challenges associated with using a fluoropolymer film as a frontsheet include providing the desired combination of barrier properties and transparency, as well as providing good adhesion to an adjacent encapsulant layer. For instance, higher transparency will improve solar module efficiency in converting sunlight into electricity, but achieving higher transparency typically requires the use of thinner fluoropolymer films, which reduces strength, weather resistance, UV resistance, and moisture barrier properties. Furthermore, the reduced barrier properties of thinner films can result in more rapid degradation of the encapsulant layer, further reducing the overall performance of the module. ETFE films have become the most widely used fluoropolymer materials for manufacture of photovoltaic (PV) module frontsheets due to the excellent adhesion of ETFE to ethylene vinyl acetate copolymers (EVA).
EVA copolymers are the most commonly used materials for the encapsulant layer, due to their low cost, high clarity, low modulus, low initial viscosity, low equilibrium moisture level, and good heat resistance. In addition, EVA copolymers have been favored encapsulant materials because they are characterized by low melting temperatures, which allows them to readily flow around and seal the solar cell components. The low melting temperature properties of EVA copolymers, however, generally necessitate subsequent crosslinking of the polymer to provide suitable thermal stability of the resulting photocells.
Moreover, the use of crosslinkable EVA as an encapsulant is not trouble-free. For example, the liberation of acetic acid from EVA can lead to corrosion and yellowing of the EVA encapsulant. Also, because peroxides are often incorporated in the EVA encapsulant as part of the crosslinking reaction, the shelf life of the EVA encapsulant is reduced and decomposition of the peroxide results in evolution of oxygen which may cause bubble formation. Finally, it is necessary that the EVA sheet be produced at a very low extrusion temperature to prevent premature crosslinking, that is, crosslinking prior to lamination to form the solar cell module.
Therefore, alternative materials that exhibit higher thermal dimensional stability without crosslinking are of interest for use in encapsulant layers.
In the past, encapsulant materials have been compounded with silane coupling agents, including aminosilanes, to improve adhesion to fluoropolymer layers. (See e.g. U.S. Pat. Nos. 6,963,120 and 6,762,508; U.S. Patent Application Publication Nos. 2009/0183773, 2009/0120489, 2009/0255571, 2008/0169023, 2008/0023063, 2008/0023064 and 2007/0267059; U.S. Patent Application Publn. No. 2011/0023943; European Patent Application No. 1065731; French Patent No. 2539419 and Japanese Patent Application Nos. 2000/186114, 2001/144313, 2004/031445, 2004/058583, 2006/032308 and 2006/190867). U.S. Pat. No. 6,753,087 describes a multilayer structure including a fluoropolymer bonded to a substrate. The structure is prepared by heating a bonding composition including an amino-substituted organosilane to form a bond. U.S. Patent Application Publication Nos. 2008/0023063, 2008/0023064, 2008/0264471 and 2008/0264481 describe solar cells in which one or both surfaces of any of the solar cell laminate layers may be treated with a silane that incorporates an amine function. U.S. Patent Application Publn. No. 2011/0297313, entitled “Method for Preparing Transparent Multilayer Film Structures Having a Perfluorinated Copolymer Resin Layer”, filed Jun. 7, 2010, and U.S. Patent Application Publn. No. 2011/0297314, entitled “Method for Preparing Multilayer Structures Containing a Perfluorinated Copolymer Resin Layer”, filed Jun. 7, 2010, describe the use of aminosilanes as surface-treatment agents or additives for ethylene copolymer films adhered to fluoropolymer films useful as PV module components.
U.S. Pat. No. 7,638,186 and European Patent Application Publication No. EP577985 describe the use of tetrafluorethylene-hexafluoropropylene copolymers, commonly referred to as FEP, as back sheet layers in photovoltaic modules. International Patent Application Publication No. WO2004/019421 describes the use of FEP as a front sheet layer in photovoltaic modules.
There is a need for alternative encapsulant materials for use in photovoltaic modules that incorporate fluoropolymer films. Such materials would desirably exhibit a combination of good adhesion to fluoropolymer layers, particularly under adverse conditions, and high light transmittance, thereby enabling development of improved photovoltaic modules.